There are eight reasons.Firstly, the mounting angle deviation is too large;secondly,there are obstruction near the collector plate;thirdly,there are much dust on the vacuums’ surface;fourthly,poor water quality leads to the accumulation of sludge within the vacuum tube;fifthly,you obeyed to the requirements to install a necessary check valve,resulting in the loss of hot water;sixth,the bad weather and lack of radiation energy;seventh,pipeline insulation processor is not in place or the insulation effect is reduced;eighth,hot water system is failing.And there are eight methods to deal.Firstly,correct the angle of installation;secondly,change the environment of installation;thirdly,clean the vacuums’ surface;;fourthly,clean the vacuum;fifthly,install the check valve;sixth,use the photoelectric complementary to aid heating;seventh,strengthening the measures of insulation;eighth,repair or replace the controller.